1. Field of the Invention
This invention relates to a fabrication method of a semiconductor. More particularly, this invention relates to a fabrication method of a photodiode.
2. Description of the Related Art
There are broad applications for a photodiode, such as photo image sensors, digital cameras, devices for video conference, and barcode readers.
For a photodiode, the photodiode dark current is the most serious factor adversely affecting the sensitivity and the performance of a photodiode. A photodiode fabricated by the conventional shallow trench isolation technique often results in high electrical field at the junction edge. This high electrical field at the junction edge will create a leakage path that generates a relatively high dark current. The occurrence of a high dark current is especially significant in a semiconductor device fabricated through the sub-micron technology, for example, 0.25 .mu.m and below.
FIGS. 1A to 1E are schematic, cross-sectional views showing the manufacturing steps for a photodiode.
Referring to FIG. 1A, a pad oxide layer 102, a silicon nitride layer 104 and a photoresist pattern 106 are sequentially formed on the substrate 100.
Referring to FIG. 1B, the substrate 100 is etched to form a trench 108 in the substrate 100, with the photoresist 106 pattern serving as a mask. An oxide layer 120 is further formed in the trench 108 filling the opening 108 and covering the silicon nitride layer 104.
As shown in FIG. 1C, the oxide layer 120a above the silicon nitride layer 104 is removed.
Referring to FIG. 1D, the silicon nitride layer 104 and the oxide pad 102 are removed. A portion of the oxide layer 120a (in FIG. 1C) is also removed, leaving the oxide layer 120b.
As shown in FIG. 1E, ion implantation is conducted on a P-well region 126, an N-well region 122, a P+ type region 124 and an N+ type region 128. The desired concentrations and depths of the implanted ion for these regions can be achieved by using the different masks and the appropriate operating conditions. FIG. 1E further includes the surroundings of the shallow trench isolation structure, which indicates the presence of more than one oxide plug 120, and the relative positions of the P well region 126, the N well region 124, the P+ type region and the N+ type region. FIG. 1E also shows a P-N junction 136. The edge 130 of the junction 136 is likely the place to have a leakage path that results in a high photodiode dark current.